1. Field of the Invention
This invention relates to jars for use in a tool string in a well and more specifically to a hydraulic jar for imparting an upward, impactive force to a tool.
2. The Prior Art
The use of a jar in a well tool string to impart an upward impactive force is well known. Jars may be either hydraulic or mechanical.
One common hydraulic jar is illustrated in U.S. Pat. No. 2,828,822 to Greer and in the "Composite Catalouge of Oil Field Service & Equipment", 1972-1973 edition Volume I page 683. This jar includes a piston in a cylinder housing having two different diameters at opposite ends.
However, the above jar has several operational and manufacturing deficiencies. Because the entire jaring stroke is done by the piston and cylinder, the tool is long. The high speed movement of the piston through the enlarged diameter portion of the cylinder wears the seals around the piston. To be able to obtain the desired retarding of fluid flow past the piston, a long elaborate packing element surrounds the piston and a small tolerance is provided between the reduced diameter portion and piston. The tolerance generally results in a lack of interchangeability of parts from one jar to another. Additionally when the jar is operated, the cylinder bore is occasionally gouged and a less restricted (e.g. faster) flow of fluid results. The manufacture of such a jar is difficult because the entire reduced diameter portion of the cylinder must be closely machined and the reduced diameter portion is in an inexcessible location for machining operations.
U.S. Pat. No. to Greer 2,851,110 also discloses a hydraulic jar. The jar is similar to that disclosed in the aforementioned patent except that instead of having an enlarged diameter portion, the jar has a cylinder having longitudinal grooves at one end through which fluid may flow around the piston. The jar also has the disadvantages of the aforementioned jar.
Mechanical jars including a spring and a hammer which is released to strike on anvil when the spring is stretched a selected extent are known. The major disadvantage with these types of jars is that there is no way of changing the amount of impactive force obtained from the jar while the jar is being used in the well. This is because the spring must be preset before the jar is positioned within the tool string. In use, the hammer is released when the spring is stretched this preset amount.